World's oldest dinosaur embryo bonebed yields organic remains

This is a flesh reconstruction of embryonic dinosaur inside egg. Credit: Artwork by D. Mazierski

The great age of the embryos is unusual because almost all known dinosaur embryos are from the Cretaceous Period. The Cretaceous ended some 125 million years after the bones at the Lufeng site were buried and fossilized.

Led by University of Toronto Mississauga paleontologist Robert Reisz, an international team of scientists from Canada, Taiwan, the People's Republic of China, Australia, and Germany excavated and analyzed over 200 bones from individuals at different stages of embryonic development.

"We are opening a new window into the lives of dinosaurs," says Reisz. "This is the first time we've been able to track the growth of embryonic dinosaurs as they developed. Our findings will have a major impact on our understanding of the biology of these animals."

The bones represent about 20 embryonic individuals of the long-necked sauropodomorph Lufengosaurus, the most common dinosaur in the region during the Early Jurassic period. An adult Lufengosaurus was approximately eight metres long.

These are animations showing the locality on Google Map, in Yunnan Province, Southwest China; Locality photo from a distance, from close up showing part of the field crew; close up of embryonic limb bone as it is preserved in the sediments; reconstruction of embryo in the egg, preserved skeleton; focus on femur; thin section of largest known femur, with exquisitely preserved internal structures, including organic remains. Credit: D. Mazierski, P. Gilston, D. Scott, R. Reisz, A. LeBlanc, S. Modesto.

The disarticulated bones probably came from several nests containing dinosaurs at various embryonic stages, giving Reisz's team the rare opportunity to study ongoing growth patterns. Dinosaur embryos are more commonly found in single nests or partial nests, which offer only a snapshot of one developmental stage.

To investigate the dinosaurs' development, the team concentrated on the largest embryonic bone, the femur. This bone showed a consistently rapid growth rate, doubling in length from 12 to 24 mm as the dinosaurs grew inside their eggs. Reisz says this very fast growth may indicate that sauropodomorphs like Lufengosaurus had a short incubation period.

This image released by the University of Toronto shows a preserved femora, thigh bone, in cross section in middle of the shaft. The purple color is caused by the lamba filter used for effective visualization. The honeycomb like external area is embryonic bone tissue with large primary spaces for blood vessels, bone making cells called osteoblasts, and other soft tissues needed for growth. The central portion is the medullary cavity, but in this case filled with crystals that formed during fossilization. An international team of scientists discovered a cache of dinosaur embryos near the city of Lufeng, in Yunnan, China . Estimated to be 190 million years old, the fossilized bones are among the oldest dinosaur embryos in the world. Credit: University of Toronto, A. LeBlanc

Reisz's team found the femurs were being reshaped even as they were in the egg. Examination of the bones' anatomy and internal structure showed that as they contracted and pulled on the hard bone tissue, the dinosaurs' muscles played an active role in changing the shape of the developing femur. "This suggests that dinosaurs, like modern birds, moved around inside their eggs," says Reisz. "It represents the first evidence of such movement in a dinosaur."

The Taiwanese members of the team also discovered organic material inside the embryonic bones. Using precisely targeted infrared spectroscopy, they conducted chemical analyses of the dinosaur bone and found evidence of what Reisz says may be collagen fibres. Collagen is a protein characteristically found in bone.

This image released by the University of Toronto shows a close-up of embryonic humerus, as it is preserved in the sediments. An international team of scientists discovered a cache of dinosaur embryos near the city of Lufeng, in Yunnan, China. Estimated to be 190 million years old, the fossilized bones are among the oldest dinosaur embryos in the world. Credit: University of Toronto, R. Reisz
"The bones of ancient animals are transformed to rock during the fossilization process," says Reisz. "To find remnants of proteins in the embryos is really remarkable, particularly since these specimens are over 100 million years older than other fossils containing similar organic material."

Only about one square metre of the bonebed has been excavated to date, but this small area also yielded pieces of eggshell, the oldest known for any terrestrial vertebrate. Reisz says this is the first time that even fragments of such delicate dinosaur eggshells, less than 100 microns thick, have been found in good condition.

"A find such as the Lufeng bonebed is extraordinarily rare in the fossil record, and is valuable for both its great age and the opportunity it offers to study dinosaur embryology," says Reisz. "It greatly enhances our knowledge of how these remarkable animals from the beginning of the Age of Dinosaurs grew."

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